I think this is mistaking the costs of publishing with the stability of knowledge.
The knowledge was never stable, even back then. The encyclopedias could only afford a team of some fixed size, to publish an edition every few years.
Wikipedia just scales that up to many editors doing real time edits. Arguably it's a more reflective representation of how organic knowledge transfer actually happens.
Citing any encyclopedia or other general reference text as a source is a warning sign, because it means that you haven't done any in-depth research, and/or you're trying to cite sources for information which is considered general knowledge in the field. Wikipedia isn't special in this regard; you're going to get marked down just as much if you cite the Encyclopedia Britannica or a dictionary.
It's a secondary or tertiary source that does a great job or collecting and summarizing primary lit (which can be cited if more rigor is preferred, at the cost of context). My college professors encouraged the use of both, maybe with the caveat of citing a particular Wikipedia revision rather than the article directly.
But seriously, these were the discussions we've had two decades ago. If a professor today had such an issue, they're very old fashioned, and I'd probably walk out of that class because who knows what else that professor is out of date on.
So many fields today are evolving so rapidly, I wouldn't trust any single expert on a topic. Better to have a living crowdsourced reference that collects many sources.
The knowledge was never stable, even back then. The encyclopedias could only afford a team of some fixed size, to publish an edition every few years.
Wikipedia just scales that up to many editors doing real time edits. Arguably it's a more reflective representation of how organic knowledge transfer actually happens.